Rooms and factories installed with industrial machines or dangerous zones around the industrial machines themselves require installation of systems for locking the machines when doors of the doorway of the room and dangerous zones are not completely closed, for the purpose of preventing occurrence of troubles such as worker injury caused by the worker being caught in the machines. In order to meet such a requirement, a safety switch (Japanese Unexamined Patent Application Publication No. 10-69831) has been proposed. The safety switch is configured as follows. A switch body of the safety switch is installed on wall surfaces in the vicinity of the doorway of the room. When the door of the doorway is closed, a dedicated actuator fixed to the door is inserted into an insertion opening of the switch body. The insertion leads to rotation of a drive cam in the switch body. An operating rod moves in response to the rotation of the drive cam so that the connection state of a contact point is switched. Such a switching operation of the contact point allows a circuit connection to be switched to a main circuit (a power supply circuit for the industrial machines) side so that the machines in the room become operable state.
However, such a safety switch has a following drawback. In the cases where a worker enters the room for maintenance or the like of the machines while keeping the machines stopped and where another worker, who does not know the fact that the maintenance is being carried out, closes the door, the machines could be started and the worker already in the room could fall into a dangerous situation.
Therefore, in order to prevent the occurrence of the above situation or the like and further to ensure safety of workers, the safety holder shown in FIG. 33 has been proposed. This safety holder 50 includes a base body 51 and a slide body 52 movably attached to the base body 51. How to use the safety holder 50 will be described below with reference to FIG. 34. Firstly, the base body 51 is inserted into an actuator insertion opening 54 of a switch body 53, whereby a protruding portion (not shown) provided on a lower surface of a tip end portion of the base body 51 engages with the actuator insertion opening 54. The slide body 52 is then moved along the base body 51 and inserted into the actuator insertion opening 54. At this time, the slide body 52 is maximally inserted all the way into the slot 54. This causes the slide body 52 to be fitted in the concave portion of a drive cam in the switch body 53 and prevents the drive cam from rotating. In this state, a lock mounting opening 55 formed in the base body 51 and a lock mounting opening 56 formed in the slide body 52 are overlapped each other, and a worker then locks each of the lock mounting openings 55 and 56 using padlocks 57, respectively. This blocks the actuator insertion opening 54, and prevents the rotation of the drive cam even though an actuator is inserted from the other actuator insertion openings 58. Therefore, the machines are prevented from being actuated accidentally and safety of workers is ensured. This safety status is maintained until the padlocks are unlocked after completion of work, which ensures safety of workers.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 10-69831